1. Field of the Invention
The present invention relates generally to a universal joint yoke having a higher stiffness and greater joint angle and which is produced by folding a sheet metal blank. The present invention relates more specifically to the above-described universal joint yoke for a steering system including two ear portions which are provided with bores for mounting bearings of two trunnions of a cross member and which are interconnected to a base portion provided with other bores through which means for securing the yoke to shock absorbing means, e.g., a rubber coupling is inserted.
2. Description of the Prior Art
Such a kind of universal joint yoke has been used as one of components constituting a steering column of a steering system applied to automotive vehicles.
FIGS. 1 and 2 show a conventional universal joint yoke.
As shown in FIGS. 1 and 2, a lower joint 3 joints a steering column 1 to a steering gear unit 2. Two yokes 5, 6 are attached respectively to upper and lower ends of a lower shaft 4 and two yokes 8, 9 are respectively attached to one end of the steering column 1 and to one end of a gear shaft 7. The respective ends of yokes 8, 9 adjoin the opposing yokes 5, 6. Each pair of yokes 5, 8 and 6, 9 is universally jointed together by means of respective trunnions 10a, 10b.
The lower joint 3 has a shock absorbing construction in order to damp vibrations and noises caused by an engine mounted on the vehicle, uneven road surfaces, etc., and transmitted from the steering system located below the lower joint 3 to a steering wheel via a steering shaft 1.
An essentially disc-shaped rubber coupling 11 is attached to the base portion of the yoke 5 as shock absorbing means by means of bolts and nuts 12, 13 and to the lower shaft 4 (to be described later). Therefore, most of the above-described vibrations and noises transmitted to the lower joint 3 are damped by means of the rubber coupling 11, so that such vibrations and noises cannot be transmitted to the steering wheel and vehicle compartment.
Japanese Utility Model Application Unexamined Open No. Sho. 57-172,930 (corresponding to U.S. Pat. No. 3,901,048 issued on Aug. 26, 1975) exemplifies the universal joint yoke in which the yoke is fixed to the lower joint 3.
In details, as shown in FIGS. 3 and 4, the yoke 5 is made of an essentially diamond-shaped metal plate with both ends of a longer diagonal folded upward to form the shape of a letter "C" in section. The yoke 5 comprises: (a) the base portion 15 having holes 14a, 14b for attaching the whole yoke 5 to the rubber coupling 11 and (b) the two ear portions 17a, 17b having two opposing holes 16a, 16b for mounting bearings of two trunnions 10a of the cross member. It is noted that the two opposing holes serve to journal the trunnion 10a.
The yoke 5 formed as described above is fixed to the upper end of the lower shaft 4 via the rubber coupling 11, as shown in FIG. 2. In this case, the rubber coupling 11 is fixed to both extended ends of mounting flangs 18a, 18b of a yoke 18 which is attached around an intermediate periphery of the lower shaft 4, the mounting flanges thereof being extended outwardly at a given angle with respect to an axial direction of the lower shaft 4. The yoke 5 is fastened to the rubber coupling 11 in such a way that a lower surface of the base portion 5 rests on an upper surface of the rubber coupling 11. Bolts 19 are inserted through the holes 14a, 14b shown in FIG. 3 and fastened to the rubber coupling 11 together with corresponding nuts as shown in FIG. 2.
It is noted that two bolts 12 are aligned on the larger diagonal of the yoke, i.e., diamond-shaped sheet metal and that the nuts 13 engaged on threaded portions of both bolts 12 are placed outside of the ear portions 17a, 17b which face each other. It is also noted that a line connecting both bolts 19 is orthogonal to the line connecting both bolts 12. The yoke 5 thus fixed to the lower shaft 4 via the rubber coupling 11 is then universally jointed to the opposing yoke 8 fixed to the steering shaft 1 via the trunnion 10a.
However, there are several drawbacks to the above-described conventional universal joint yoke.
Specifically, since the conventional yoke 5 is formed merely by folding the essentially diamond-shaped metal plate along parallel fold lines to define the base 15 and two ear portions 17a, 17b, the stiffness in the region of fold lines of the two ear portions 17a, 17b is so low that, when the yoke 5 is incorporated in the universal joint of the steering column and a high steering torque is subjected to the yoke 5, the ear portions 17a, 17b tend to deform outwardly.
In addition, since the rubber coupling 11 is fixed to the mounting flanges 18a, 18b by means of the bolts 12 and nuts 13, ends of the nuts 13 must be located outside the outer surfaces of the respective ear portions 17a, 17b so that the bolts 12 and nuts 13 do not come into contact with the ear portions during the movement of universal joint with the steering wheel. Therefore, the outer dimension of the rubber coupling 11 must inevitably be increased in order to avoid the ends of nuts 13 and bolts 12 from coming into contact with the ear portions 17a, 17b and to render the rubber coupling 11 have a strength enough to withstand the torsional torque applied through the bolts 13 without increase in thickness of the yoke since the dimensions of the yoke are usually determined according to respective vehicle models.
Consequently, a rotation radius of the rubber coupling 11 is accordingly increased.
In this case, a joint rotation radius is similarly increased and outer appearance of the universal joint is unsightly.
Furthermore, since moderately inclined surfaces 20a, 20b are formed on both side edges of each ear portion 17a, 17b as shown in FIG. 4, a working angle of the universal joint (the joint angle of the yoke 5 with respect to the opposite yoke 8) is consequently restricted. Therefore, a spatial utility of the steering shaft 1 as well as the whole steering system is accordingly reduced.